Electro-weldable sleeves are designed in particular to make a connection between thermo-plastic tubes, e.g. tubes constituting pipework for distributing gas or water or other fluids and made, in general, of polyethylene.
The conventional electro-welding technique consists in heating contacting surfaces of the pieces by means of an electrical resistance heater wire embedded in a coupling, thereby reaching the welding temperature of the materials used. In general, such couplings are made by injection molding over the heater wire.
Unless otherwise specified, the term "plastic" is used herein in its commonly-accepted broad sense of "thermo-plastic material", and in particular it is used to cover such material when in its hard state, i.e. when it is not plastic in the narrow sense of the word.
Over the last ten years or so, Japanese and American laboratories have developed revolutionary materials constituted by intrisically conductive polymers. These polymers are conductive because of doping ions (iodine, sodium, mercury, . . . ) included therein that facilitate the free displacement of electrons along the chain, i.e. the conduction of electricity.
In addition to their own characteristics (mechanical qualities, lightness, ease of implementation, manufacturing cost, . . . ), these plastics also provide electrical conductivity to a degree which can be adjusted by modifying the concentration of dopant.
Several applications have since been developed. Firstly, these materials can be used to store energy in electric cells or batteries made entirely of plastic or having electrodes made of plastic. A battery cell having polyaniline electrodes is now commercially available. Another intended application is the manufacture of display screens that make use of the change in color that occurs in some polymers when subjected to an electrical voltage. Another line of development relates to a conductive molecular thread made of polymer and used as a chain for transferring information between an electrode and sensitive functional groups grafted on the thread.
More conventional extrinsically conductive plastics have also been developed which contain a filler of metal fibers or powder, or of carbon black.
French patent number 2 586 036 (Kao Corporation and Yoshino) describes a method of preparing an intrinsically conductive polymer. In this method, an electrolyte and a monomer in the form of an aromatic compound or of a derivative thereof are dissolved or dispersed in a polar organic medium, and electro-chemical anode oxidation is performed in the resulting solution or dispersion in the presence of at least one metal ion, the said metal ion being selected from: copper, bivalent silver, trivalent iron, trivalent manganese, ruthenium, rhenium, and rhodium.
French patent number 2 586 133 (Protavic) describes an improved extrinsically conductive polymer composition containing finely-divided nickel. In such compositions, electrical conductivity is essentially due to metal particles establishing mutual electrical contact between one another, which particles may be silver, nickel, aluminum, copper, . . .
The present invention relates to a novel application of such extrinsically or intrinsically conductive plastics. It relates to a method of making electro-welding couplings by using an electro-welding insert made of such plastics.
The main advantage of the method lies in its ease of implementation and in its low manufacturing costs. As demonstrated by the numerous patents filed in this field, serious problems of reliability and welding quality arise with the installation of a conventional helical heater wire in or on a plastic former.
The main drawback of such methods lies in the problem of the risk of the heater wire expanding and moving both before and during overmolding.
Prior to overmolding, the wire is held in place solely by being under tension around the surface of the former. However, this tension may be faulty, for example due to an operator error during winding or during storage because of a change in the sleeve's diameter due to ambient atmospheric conditions, in particular excessive humidity.
In addition, it is difficult to adjust the tension to be applied to the wire while it is being wound on the grooved sleeve. The wire must be under considerable tension in order to be received reliably in the bottom of the helical groove. However, the wire must not be under excessive tension since, in that case, its diameter is reduced and the resistance finally obtained is greater than that which is desired, thereby giving rise to problems during electro-welding.
The wire may also move during overmolding because of forces due to the viscosity of the plastic being overmolded.
These displacements, if they occur, reduce the quality of the weld, giving rise to non-uniform heat distribution. They also run the risk of causing pockets of air to be created in the coupling, and these too reduce weld quality.
A heating insert may also be used inside a coupling for the purpose of making a branch connection via a hole made in situ in a duct.
Such couplings are generally made in two portions which receive the duct. The hole is made after welding by heating the duct in the zone where the hole is to be made, with the heating being obtained by means of a heating electrical resistance.
In the same manner as for a connection sleeve, attempts have been made to provide a prefabricated heating insert for such a branch coupling.
In the prior art as described, for example, in French patent number 2 171 223 (Rollmaplast AG), an insulated resistor wire in the form of a spiral or in some other form is embedded in the bulk of a saddle-shaped portion by overmolding.
In contrast, a prefabricated insert is described in French patent number 2 519 578 (Innovation Technique Sam). This insert comprises a relatively thin molded plate including a groove in which a spiral-shaped heater wire is installed. The insert is intended to be incorporated in the inside face of the saddle, preferably while the saddle is being molded.
As shown by these prior documents, installing a spiral heater wire in this case also gives rise to problems of reliability and welding quality.
The above-mentioned problems of heater wire displacement, expansion, and adjustment in electro-welding formers apply in this case as well.
The method of the invention lends itself to making a coupling which is self-fusing, i.e. a very simple disposition causes the flow of electricity to be interrupted automatically when the energy required for welding has been dissipated.
In prior electro-welding connections, devices for interrupting the flow of electricity are relatively complex.
According to French patent number 2 572 327 (Toutelectric Sam MC), a device for controlling welding time includes a relay connected in a main circuit for manual switching and for switching under the control of a microswitch itself controlled by the pressure of the molten substance expanding during the welding operation. Such microswitches are disposed in wells close to the power supply terminals and hollowed out in the wall of the sleeve or the saddle down to the vicinity of the windings. This device for controlling welding time is adaptable to sleeves for connecting tubular elements and also to saddles for making a branch connection by making a hole in a tubular element.
According to European patent number 229 743 (Geberit AG) the coupling is provided, adjacent to each terminal, with a sleeve containing a temperature indicator in the form of a thin disk having a color coating whose color changes with temperature. This temperature indicator co-operates with an optical sensor which switches off the power supply when the disk takes on a determined color.
According to European patent number 93 821 (Geberit AG), the conductor wire is provided with a switch designed to break the wire when the energy required for welding has been transmitted.
Unlike these complex arrangements for interrupting electro-welding, the present invention proposes a particularly simple system for thermal regulation of the weld, which system requires no additional device to be installed and is capable of being implemented simply during molding of the electro-welding insert since the thermal regulator elements are constituted by integral portions of the insert.